Im really sorry for you and your sister.
As a terminal myself, i can understand what your sister is experiencing. I had a period of confusion, trying to assess what is happening to me, what should i be doing, should I fight the disease?, should I simply accept the inevitable, set my life straight, enjoy the remaining moments and plan for pain management program, hospice care, etc so my death will still be atleast dignified?.
During this period, I refrain from talking to my family. I specially don't like being told what to do. Cancer pretty much takes away my ability to control my life and I still want some control over a certain aspect of it.
I guess your sister is undergoing the same process. You can try giving her some encouragement as some terminal people feel very helpless and unable to make decision for themselves. Provide her the assurance that you will support (and help her on) whatever decision she will make.